This application is a continuation of pending U.S. patent application Ser. No. 12/633,139, filed Dec. 8, 2009.The invention is directed to transmission systems such as optical transmission systems and, more particularly, to detecting, identifying, and/or measuring impairments present in signals carried by such transmission systems.
Presently, performance of an optical transmission system is monitored using methods that can determine the existence of an anomaly in the transmission system but which cannot identify the cause of the anomaly. Specifically, frequency domain (spectral) methods are used and monitor, for example, optical channel power, total dense wavelength division multiplexing (DWDM) signal optical power, optical signal-to-noise ratio (OSNR), and/or optical channel wavelength. The known spectral methods, however, are merely averaging methods and cannot sense a pulse or its duration, and thus the known spectral methods are insensitive to effects caused by such pulse distortions.
The known spectral methods, therefore, are best suited for static point-to-point wavelength division multiplexing (WDM) systems. Dynamic WDM (DWDM) systems, by contrast, frequently change the light path connections that are used to convey the optical signals. Each time the light path connections are changed, the performance of the DWDM system can change. Because the known spectral methods are averaging methods, these methods are unsuitable for monitoring a dyhamic WDM system.
Other, more sophisticated monitoring techniques that measure time domain signal properties, such as analysis of captured eye diagrams, Q-factor analysis, etc., are capable of detecting pulse distortions. Such techniques, however, are carried out using expensive test equipment, such as sampling oscilloscopes or Q-factor meters, and hence are not cost effective.
Still other experimental monitoring techniques have focused on measuring the error performance, such as the bit error rate (BER), as detected at a receiver. Such techniques, for example, carry out sampling at an optimal point in an eye diagram to get a good quality estimate of the bit error rate. These techniques, however, have only a limited ability to identify a type of impairment that is causing a change in the optical transmission systems performance. That is, such techniques have only a limited ability to detect impairments such as chromatic dispersion, polarization mode dispersion, cross-talk, etc.
It is therefore desirable to provide performance monitoring that can identify abrupt fault conditions in the optical transmission system and the causes of such abrupt fault conditions. It is also desirable to provide performance monitoring that can identify any small changes or trends in the conditions of the optical transmission system and thus detect the precursors of a fault condition. It is further desirable to provide performance monitoring that can identify the cause of such small changes or trends, thereby allowing remedial action to be taken before the fault condition materializes.